One form of detector used in fire alarm systems is a heat detector. Such detectors generally trigger an alarm not only when the actual temperature reaches some predetermined level but also when the rate of rise of temperature exceeds some level. A rapid rise in temperature can provide an early indication of some fire conditions.
Underwriters Laboratories rates heat detectors according to how quickly they respond to a rapid fire situation (UL 521). In order to minimize the number of detectors required in an environment, the detector should respond to a rapid fire condition in less than 30 seconds, and preferably less than 20 seconds. On the other hand, the detector should not be so sensitive that it indicates a fire condition at less than 130.degree. F. where the rate of rise of temperature in the environment is no faster than 12.degree. F. per minute.
The conventional heat detector includes a bimetallic temperature sensor mounted relative to a diaphragm on which an electrical contact is positioned. The diaphragm responds to a change in pressure between two chambers resulting from rapid changes in temperature of the air within the chambers. Such detectors are well understood and are reasonably accurate and inexpensive. However, they do suffer the expected changes in tolerance of mechanical components with age.
More recently, electronic heat detectors have been introduced. Such detectors respond to the difference in outputs of one temperature sensor exposed to the monitored environment and a reference temperature sensor mounted within the heat detector housing. To meet the Underwriters Laboratories tests, the expense of the required electronics has been such that the electronic heat detectors have not been price competitive with conventional mechanical detectors.